Up to now, there has been known a disc device that processes a disc-shaped recording medium such as an optical disc by the aid of an optical pickup as an information processor that reads information that has been recorded on a recording surface or records various types of information on the recording surface. The known optical pickup of the disc device is configured to control a position to which an optical beam is irradiated on the basis of the optical beam that has been outputted from a light source and reflected by the optical disc, for example, to implement the tracking servo control of an objective lens. As the tracking servo control, there has been known a configuration that diffracts the light beam that is outputted from the light source to detect a focal position (refer to, for example, Patent Document 1).
In the configuration disclosed in Patent Document 1, a laser beam is split into a zeroth-order light beam and ±first-order diffracted light beams by means of a diffraction grating. Further, the respective light beams are converged so that the intervals of three convergent spots (hereinafter, of those three convergent spots, the central convergent spot will be referred to as “a main spot”, and convergent spots at both sides of the main spot will be referred to as “side spots”) that are formed on the recording surface of the optical disc substantially coincide with ½ of a cycle of guide grooves that are defined in the optical disc. After that, the optical disc reflected light beams of the respective spots are received by 3/2 divided or ¾ divided light receiving surfaces, and subjected to subtraction processing by means of a subtracter, thereby detecting a push-pull signal Sa of the main spot, and push-pull signals Sb and Sc of the respective sub-spots. Then, when the push-pull signals Sb and Sc of the respective side spots are subjected to addition processing, tracking error components within the respective signals are canceled, and only offset components are added and outputted as a signal Sd as in the conventional DPP (differential push pull) system. After that, the signal Sd is amplified by a given gain, and then subtracted from the push-pull signal Sa, thereby obtaining a configuration that detects an excellent tracking error signal in which only the offset components are removed from the signal Sa.
[Patent Document 1] JP-A 2003-272190 (right column of p. 4 to left column of p. 7)